1. Field of the Invention
The disclosures herein generally relate to an electrochromic compound, an electrochromic composition, and a display device.
2. Description of the Related Art
Recently, efforts are being directed to developing electronic paper as an electronic medium to be used in place of paper. The electronic paper may be embodied by a display device that is designed to be used just like ordinary paper. Such a display device desirably has characteristics that are different from those of traditional display devices such as a CRT (Cathode Ray Tube) or an LCD (Liquid Crystal Display). For example, the electronic paper may require a display device to have the characteristics of a reflective display, to have a high white reflectivity and a high contrast ratio, to display high-resolution images, to have an adequate memory effect, to operate even with a low voltage input, to be thin and light-weight, and to be inexpensive. The electronic paper has particularly demanding requirements with regard to color display characteristics and display quality characteristics such as having the same white reflectivity and contrast ratio as those of paper.
There have been disclosures related to display techniques adopted by a display device for the electronic paper including a method of using a reflective liquid crystal, a method of using electrophoresis, and a method of using toner migration, for example. However, it is difficult to display multiple colors while maintaining a high white reflectivity and a high contrast ratio when using the above disclosed techniques. To display multiple colors, a color filter is conventionally used. The color filter, however, may absorb light, and as a result, the reflectivity of the display device may be reduced. Also, by using the color filter, one pixel is divided into three regions (i.e., red (R), green (G), and blue (B) regions). Therefore, the reflectivity of the display device may be reduced and the contrast ratio may be reduced accordingly. When the white reflectivity and the contrast ratio are greatly reduced, the visibility may also be degraded. As a result, it may be difficult to use the display device as electronic paper.
Japanese Laid-Open Patent Publication Nos. 2003-161964 and 2004-361514 (Patent Documents 1-2) disclose techniques related to a reflective color display medium having a color filter arranged on an electrophoresis element. However, a desirable image cannot be obtained even if a color filter is arranged on a display medium with a low reflectivity and a low contrast ratio.
Japanese Translation of PCT International Application Publication Nos. 2004-520621 and 2004-536344 (Patent Documents 3-4) disclose techniques related to an electrophoresis element that enables color display by moving particles that are in plural different colors. However, these techniques cannot fundamentally resolve the above-described problem related to the demand for a high reflectivity and a high contrast ratio. That is, the above techniques cannot achieve a high white reflectivity and a high contrast ratio at the same time.
On the other hand, there is one promising technique for the reflective display device using an electrochromic phenomenon without using a color filter. The electrochromic phenomenon is also called electrochromism and refers to a phenomenon in which electrochromic materials applied with a voltage show a reversible color change during an electrochemical redox reaction. An electrochromic display device, which utilizes such a color change (i.e., coloration and decoloration) in the electrochromic materials that cause such a phenomenon, has emerged as a candidate for electrochromic paper because it serves as a reflective display device, has high white reflectivity as well as a memory effect, and can be driven at a low voltage.
It is noted that since the electrochromic display device utilizes the coloration/decoloration during redox reactions, the coloration/decoloration response speed of the electrochromic display device may be relatively slow. In turn, for example, Japanese Translation of PCT International Application Publication No. 2001-510590 (Patent Document 5) discloses a technique for improving the coloration/decoloration response speed by fixing (disposing) an electrochromic compound near an electrode. This document describes how the coloration/decoloration time which conventionally took a few dozen seconds can be reduced such that both the coloration time from colorless to blue and the decoloration time from blue to colorless may only take about 1 second. However, the improvement in the coloration/decoloration response speed realized by the above technique is not adequate and further improvements in the coloration/decoloration response speed are in demand.
The electrochromic display device may display various colors depending on the structure of the electrochromic compound and may be reversibly changed from a decolored state to a colored state. Thus, the electrochromic display device may be capable of multicolor display using a layered structure. By realizing multicolor display using a layered structure, degradation of the reflectivity and the contrast ratio of the display device may be prevented since one pixel would not have to be divided into three regions (i.e., red (R), green (G), and blue (B) regions) as in the conventional display device.
For example, Japanese Laid-Open Patent Publication No. 2003-121883 (Patent Document 6) discloses a multicolor electrochromic display device that has plural types of electrochromic compound particles arranged into layers. The disclosed multicolor electrochromic display device arranges multiple layers of electrochromic compounds corresponding to high polymer compounds having plural functional groups that produce color at differing voltages.
Additionally, Japanese Laid-Open Patent Publication No. 2006-106669 (Patent Document 7) discloses a multicolor display device having plural electrochromic layers arranged on an electrode, the device being configured to produce multiple colors by utilizing the differences in the voltages and electric currents required for causing coloration of the electrochromic layers. The disclosed multicolor display device includes a display layer formed by layering or mixing two or more of electrochromic compositions that develop different colors, have different threshold voltages for coloration, and require different charge amounts for achieving coloration to a sufficient color density.
Further, Japanese Laid-Open Patent Publication No. 2003-270671 (Patent Document 8) discloses a multicolor display device that has plural layers of a structural unit that is formed by arranging an electrochromic layer and an electrolyte between a pair of transparent electrodes. Japanese Laid-Open Patent Publication No. 2004-151265 (Patent Document 9) discloses a multicolor display device that uses the above structural unit to form a passive matrix panel and an active matrix panel to realize multicolor display according to the RGB color model.
The electrochromic display devices disclosed in Patent Documents 5-7 use viologen organic electrochromic compounds that are capable of developing colors such as blue and green but are not capable of developing the three primary colors, yellow (Y), magenta (M), and cyan (C) that are required for realizing full-color display.
Japanese Laid-Open Patent Publication Nos. 2008-122578 and 2011-102287 (Patent Documents 10-11) disclose the use of styryl dye to enable YMC coloration. The electrochromic display devices disclosed in Patent Documents 8-9 also use the styryl dye to enable YMC coloration.
The electrochromic display devices disclosed in Patent Documents 5-11 have problems related to coloration stability and fall short of achieving desired characteristics as an electrochromic display device. For example, in the disclosed electrochromic display devices, the memory effect is inadequate and when an applied voltage is turned off, color that has been gradually developing may disappear relatively fast.